1. Field of the Invention
The present invention relates to a Raman amplifier for amplifying signal light for optical communication, an optical communication system equipped with the Raman amplifier, and a method for controlling the Raman amplifier.
2. Description of the Related Art
Wavelength division multiplex transmission technology for transmitting a high-density information volume by amplifying signal light of a plurality of wavelengths in a batch mode is employed for creating a network of a long-distance transmission optical communication system suitable for high-volume communication.
A Raman amplifier is used for realizing a long-distance optical transmission system with excellent signal-noise characteristic in the wavelength division multiplexing transmission technology. A Raman amplifier is an amplifier that uses an optical fiber as an amplification medium by being incident a high-intensity excitation light into an optical fiber transmission channel.
The Raman amplifier uses a physical phenomenon according to which when an excitation light of a certain wavelength is incident into an optical fiber, such as shown in FIG. 1, a Raman amplification effect is produced in a long wavelength band of about 100 nm with respect to the wavelength of the excitation light. In the example shown in FIG. 1, Raman gains 1, 2, 3 are generated by the excitation lights 1, 2, 3.
In the case of quartz glass used for an optical fiber, the maximum amplification characteristic is obtained in a wavelength region less than about 13.2 THz with respect to the wavelength of the excitation light. Therefore, for example, when a signal light close to 1550 nm is Raman amplified, a Raman gain can be obtained with good efficiency if an excitation light with a wavelength of about 1450 nm is used.
In a communication system that requires signal light in a wide wavelength region, such as that of wavelength division multiplexing transmission, to be amplified in a batch mode, an amplification characteristic in a wide wavelength band corresponding to the intensity and wavelength of the excitation light can be obtained by using excitation light of a plurality of types with mutually different wavelengths and controlling the intensity of each excitation light.
Further, a specific feature of the Raman amplifier that realizes a long-distance optical transmission system is that the amplifier has the following two functions.
(1) A Raman amplification function according to which the intensity of excitation light is regulated so as to obtain the predetermined gain, while monitoring the intensity of the signal light, with the object of compensating optical loss of the signal light that takes place when the signal light passes through the optical fiber and obtaining good transmission characteristic, and
(2) a gain deviation control function according to which the intensity of excitation light corresponding to the amplification band of each excitation light wavelength is controlled by using the principle explained with reference to FIG. 1 and a deviation of the signal light intensity after Raman amplification is controlled to the predetermined characteristic in order to compensate the gain deviation of light amplifiers connected in a multistage mode and a wavelength characteristic of optical loss in the optical fiber and the like.
In Raman amplification, the Raman amplification gain of the signal light intensity corresponding to the amplification band of the excitation light wavelength is known to increase proportionally to the intensity of the excitation light incident into the optical fiber. By incident excitation light of a plurality of wavelengths into the optical fiber, it is possible to expand the amplification band of the signal light and amplify the signal light intently in a wide range in a batch mode.
As shown in FIG. 1, excitation lights 1, 2, 3 that have different wavelengths are assumed to have Raman amplification bands in the wavelength bands shown by Raman gains 1, 2, 3. When the intensity of the excitation light 1 is high, the amplification gain of the Raman gain 1 is high, when the intensity of the excitation light 2 is high, the amplification gain of the Raman gain 2 is high, and when the intensity of the excitation light 3 is high, the amplification gain of the Raman gain 3 is high.
For example, let us consider the case where the signal light intensity of the wavelength band of Raman gain 1 is higher than the signal light intensity of the bands of Raman gains 2, 3, and the signal light intensity of the wavelength band of Raman gain 3 is lower than the signal light intensity of the bands of Raman gains 1, 2. By realizing the gain deviation control function of clause (2) above, the intensity of excitation light 1 is suppressed so as to decrease the Raman gain of the signal light intensity of Raman gain 1, whereas the intensity of excitation light 3 is increased so as to increase the Raman gain of signal light intensity of Raman gain 3, and the wavelength characteristic of the signal light intensity of Raman gains 1, 2, 3 is flattened.
A method for implementing the gain deviation control of the Raman amplifier will be explained below.
A configuration example of the Raman amplifier for implementing the gain deviation control is shown in FIG. 2.
The Raman amplifier shown in FIG. 2 has a group of excitation light sources including light sources 11a, 11b, 11c of excitation light with different wavelengths in a Raman amplification unit 1, a complete signal light monitoring circuit 14 for use in controlling the Raman amplification gain, and an excitation light control circuit 15.
Referring to FIG. 2, the excitation lights of the excitation light sources 11a, 11b, 11c are multiplexed in a multiplexer 10 and outputted into an optical fiber transmission channel 2. On the other hand, a wavelength multiplexed optical signal outputted from the optical fiber transmission channel 2 is branched by a branching device 12 to a rear stage side and to the complete signal light monitoring circuit 14 and signal light monitors 21a, 21b-21n for each wavelength.
As for the detection signal that is detected by a photodetection element 13 and converted into an electric signal, the intensity of the signal light in a wide band that was optically amplified in a batch mode is monitored by the complete signal light monitoring circuit 14, and the sum total of excitation light intensities is controlled so that the output of the detection signal has a desired gain in the gain amount monitoring unit 15a of the excitation light control circuit 15.
The configuration shown in FIG. 2 additionally has a signal light branching unit 20 such as an optical branching unit of an array waveguide grating type that branches the signal light intensities with different wavelengths to monitor the deviation of signal light intensity after Raman amplification and a plurality of signal light monitors 21a, 21b-21n that monitor the light intensity of respective wavelength signals separated for each wavelength by the signal light branching unit 20.
As for the signal light intensity monitored by the signal light monitors 21a, 21b-21n, the gain deviation amount is monitored with the gain deviation monitoring circuit 22 and used for calculating the excitation light ratio of excitation light sources 11a, 11b, 11c of different wavelength of the Raman amplifier in the excitation light ratio calculation unit 15b. 
By controlling the intensity of excitation lights outputted from a plurality of excitation light sources 11a, 11b, 11c with different wavelengths of a group of excitation light sources by the output of the excitation light ratio calculation unit 15b, the signal light of an amplification band corresponding to the excitation light wavelength is Raman amplified.
Thus, with the conventional configuration shown in FIG. 2, in the excitation light control circuit 15, the gain amount found by the gain amount monitoring unit 15a from the signal light level monitored by the complete signal light monitoring circuit 14 is monitored and the predetermined Raman gain is controlled. In addition, the wavelength deviation of the signal light intensity after Raman amplification is monitored and the excitation light intensity of each wavelength is set.
In an optical transmission system in which light amplifiers are continuously connected in a multistage fashion, a gain deviation is mainly caused by the following two factors.
(a) Loss per Unit Distance of the Optical Fiber Transmission Channel
The optical fiber has a wavelength characteristic of optical loss per unit distance. Even if a signal light with an intensity deviation of zero before the transmission in the optical fiber is transmitted in the optical fiber of a large length, the intensity deviation occurs in the signal light after the transmission in the optical fiber due to optical loss in the optical fiber.
(b) Cumulative Gain Deviation of Light Amplifier
For example, a light amplifier using an erbium-doped optical fiber as an amplification medium has a gain equalizer for canceling the wavelength characteristic of amplification efficiency of the amplification medium. Further, in most cases the gain deviation per one light amplifier can be suppressed, but in a system in which light amplifiers are sequentially connected in a multistage fashion, the wavelength characteristic of gain per one device sometimes accumulates.
FIG. 3 shows the adjacent transmission and reception light amplifiers 3, 4 connected to the optical fiber serving as the transmission channel 2 in the optical transmission system in which light amplifiers are sequentially connected in the multistage fashion.
FIG. 4 (FIGS. 4A-4D) shows spectra of signal light where intensity deviation has occurred due to the above-mentioned factors (a) and (b) in the connection diagram of the light amplifiers 3, 4 and optical fiber 2 shown in FIG. 3.
FIG. 4A shows the amplification efficiency of the amplification medium of the transmission light amplifier 3 and the intensity of the output signal light of the transmission light amplifier 3 obtained after the light has passed through a gain equalizer that cancels the gain deviation.
FIG. 4B shows a wavelength characteristic of optical loss of the optical fiber 2 that transmits the signal light.
FIG. 4C shows the amplification efficiency of the amplification medium of the reception light amplifier 4 and the gain deviation of the reception light amplifier 4 obtained after the light has passed through the gain equalizer that cancels the gain deviation.
FIG. 4D shows the deviation of signal light intensity after the light has passes through all the reception and transmission light amplifiers 3, 4 and optical fiber 2 of FIGS. 4A-4C. FIG. 4D shows a mode in which as a result of combining wavelength characteristics of gain deviation of light amplifiers 3, 4 and optical loss of the optical fiber 2, the intensity of signal light increased at a wavelength close to the center of the signal band, but the intensity of signal light at the small wavelength side is decreased with respect to the average intensity.
FIG. 5 (FIG. 5A to FIG. 5C) shows an example of the gain deviation control that flattens the wavelength characteristic of the intensity of signal light that accumulated as described hereinabove.
FIG. 5A shows a wavelength arrangement and spectrum of signal light in the initial state (a) and the intensity of excitation light of the Raman amplifier (b) in the upper and lower sections of the figure, respectively.
In the initial state, a signal light (FIG. 5A, (a)) is obtained in which intensity deviation is inhibited by the intensity of excitation light (FIG. 5A, (b)) provided from the Raman amplifier.
FIG. 5B shows a spectrum of the signal light in which the signal light at a short wavelength side was augmented from the initial state and an intensity deviation has occurred. The intensity of excitation light of the Raman amplifier at this time does not change with respect to that of the initial state shown in FIG. 5A (FIG. 5B, (b)). A state in which due to the factors shown in FIGS. 4A to 4D, a signal light with an intensity lower than the average intensity of signal light has been added to the small wavelength side and a deviation has occurred in the intensity of signal light is shown in FIG. 5B, (a).
FIG. 5C shows a spectrum of signal light in which the wavelength characteristic of the signal light intensity was flattened by further implementing the gain deviation control from the state shown in FIG. 5B (FIG. 5C, (a)) and the intensity of excitation light of the Raman amplifier (FIG. 5C, (b)).
In the gain deviation control, the intensity of excitation light of the excitation lights 1, 2, 3 or the intensity ratio of excitation lights is varied so as to inhibit the deviation of the signal light intensity after Raman amplification. As a result, the intensity of the excitation light 1 is raised and the Raman gain of the short-wavelength signal is increased, whereas the intensity of excitation lights 2, 3 is decreased and the Raman gain of signals of other wavelengths is inhibited (FIG. 5C, (b)).
By implementing the gain deviation control explained with reference to FIG. 5A to FIG. 5C, a good transmission characteristic with a small intensity deviation of signal light of each wavelength can be obtained even in an optical transmission system with a multistage connection.
It was suggested to employ a configuration that uses excitation light sources of a plurality of wavelengths when a signal light of a wide band is Raman amplified by using a Raman amplifier. The inventions described in Japanese Patent Applications Laid-open No. 2000-98433, 2001-7768, 2002-72262 are known as the inventions relating to gain deviation control of a Raman amplifier using excitation light sources of a plurality of wavelengths.
FIG. 6 (FIG. 6A, FIG. 6B) illustrates a problem arising when the gain deviation control is implemented by using the invention described in Japanese Patent Application Laid-open No. 2000-98433.
FIG. 6A, (a) shows a state in which a signal light with an intensity lower than the average intensity of signal light is on the short wavelength side and long wavelength side, but a signal light with a high intensity is close to the center of the wavelength band, and a gain deviation occurs.
In this case, FIG. 6B shows the excitation light intensity (b) and signal light intensity (a) obtained by performing Raman amplification after implementing the gain deviation control from the state shown in FIG. 6A.
A specific feature of the invention described in Japanese Patent Application Laid-open No. 2000-98433 is that the gain deviation control is implemented separately for two groups with different wavelengths of excitation light source, and the control increases the excitation light power of one group and decreases the excitation light power of the other group.
As a result, in Raman amplification having an amplification band corresponding to the wavelength of the excitation light, the deviation of signal light intensity can be eliminated when the signal light intensity increases monotonously or decreases monotonously with respect to the signal light wavelength.
However, in the case where a signal light with a low intensity is on the short wavelength side and long wavelength side, but a signal light with a high intensity is close to the center of the wavelength band as shown in FIG. 6A, the intensity deviation shown in FIG. 6A cannot be eliminated, as shown in FIG. 6B, and gain deviation control is impossible.
FIG. 7 illustrates a problem arising when a gain deviation control is implemented by using the invention described in Japanese Patent Application Laid-open No. 2001-7768.
The gain deviation occurrence state shown in FIG. 7A is identical to that shown in FIG. 6A. FIG. 7B shows the excitation light intensity (FIG. 7, (b)) and signal light intensity obtained by performing Raman amplification after implementing the gain deviation control from the state shown in FIG. 7A.
The invention described in Japanese Patent Application Laid-open No. 2001-7768 also includes a configuration divided into three or more groups according to the excitation light wavelength, and when a gain deviation control is performed, the excitation light power of at least one wavelength band is fixed and the excitation light power of the other wavelength bands is controlled. For example, in a Raman amplifier composed of three or more groups of excitation light sources, a control is performed such that when the power of excitation light 1 is fixed and the power of excitation light 2 and excitation light 3 is regulated due to the deviation of signal light intensity having an extremum point shown in FIG. 7A, the intensity of excitation light 2 is decreased, whereas the intensity of excitation light 3 is increased.
In this case, as shown in FIG. 7B, the intensity is increased by the excitation light 3 and the intensity deviation obviously remains.
Further, FIG. 8 (FIG. 8A, FIG. 8B) illustrates a problem arising when a gain deviation control is implemented by using the invention described in Japanese Patent Application Laid-open No. 2002-72262.
The gain deviation occurrence state shown in FIG. 8A is also identical to that shown in FIG. 6A. FIG. 8B shows the excitation light intensity and signal light intensity obtained by performing Raman amplification after implementing the gain deviation control from the state shown in FIG. 8A.
The invention described in Japanese Patent Application Laid-open No. 2002-72262 also includes a configuration divided into three or more groups according to the excitation light wavelength. With such control, the variation amount of the power of each excitation light that eliminates the gain deviation is calculated and adjusted from matrix elements that increase and decrease the power of the excitation light of each wavelength according to the intensity deviation from the average signal intensity.
For example, in a Raman amplifier composed of three or more groups of excitation light sources, with the deviation of signal light intensity that has an extremum point shown in FIG. 8A, the gain deviation is eliminated by increasing the power of excitation lights 1, 3 and decreasing the power of excitation light 2.
With the invention described in Japanese Patent Application Laid-open No. 2002-72262, the wavelength characteristic can be flattened by eliminating the intensity deviation of individual signal lights, but the average power of the entire configuration fluctuates.
Thus, all the inventions described in Japanese Patent Applications Laid-open No. 2000-98433, 2001-7768, 2002-72262 and serving as prior art have the aforementioned problems associated with the gain deviation control performed to flatten the wavelength characteristic of signal light intensity.
Thus, in the invention described in Japanese Patent Application Laid-open No. 2000-98433, in Raman amplification having an amplification band corresponding to the wavelength of excitation light, the deviation of signal light intensity can be eliminated when the signal light intensity tends to increase monotonously or decrease monotonously with respect to the signal light wavelength. However, in the case of deviation of signal light intensity that has an extremum point shown in FIG. 6A, the deviation of signal light intensity is difficult to eliminate.
Further, in the invention described in Japanese Patent Application Laid-open No. 2001-7768, in the Raman amplifier composed of three or more groups of excitation light sources, the Raman gain in the vicinity of the center of the signal light band corresponding to the excitation light 2 is inhibited, whereas the Raman gain corresponding to the excitation light 3 is increased, whereby the deviation of signal light intensity at the short wavelength side of the wavelength band and close to the center thereof is eliminated, but the signal light intensity at the long wavelength side is increased and the deviation of signal light intensity cannot be eliminated.
Further, in the invention described in Japanese Patent Application Laid-open No. 2002-72262, the variation amounts of excitation lights 1, 2, 3 are set, but the sum total thereof is not controlled. The resultant problem is that the average signal light intensity before the gain deviation control is different from that after the control.